Metal-air batteries use a negative electrode based on a metal such as zinc, iron or lithium, coupled to an air electrode. The electrolyte used most often is an alkaline aqueous electrolyte.
During discharging of such a battery, oxygen is reduced at the positive electrode and the metal is oxidized at the negative electrode:                Discharge at the negative electrode: M→Mn++n e−        Discharge at the positive electrode: O2+2 H2O+4 e−→4 OH−        
When a metal-air battery has to be recharged electrically, the direction of the current is reversed. Oxygen is produced at the positive electrode and the metal is redeposited by reduction at the negative electrode:                Recharge at the negative electrode: Mn++n e−→M        Recharge at the positive electrode: 4 OH−→O2+2 H2O+4 e−        
Metal-air systems have the advantage of using a positive electrode of infinite capacity. Electrochemical generators of the metal-air type are therefore known for their high specific energies, which can reach several hundred Wh/kg. The oxygen consumed at the positive electrode does not need to be stored in the electrode but can be taken from the surrounding air. Air electrodes are also used in alkaline fuel cells, which are particularly advantageous compared with other systems owing to the high reaction kinetics at the level of the electrodes and owing to the absence of noble metals such as platinum.
Much work has been carried out over several decades for the development and optimization of air electrodes.
An air electrode is a porous solid structure in contact with the liquid electrolyte. The interface between the air electrode and the liquid electrolyte is a so-called “triple-contact” interface, where the active solid material of the electrode, the gaseous oxidant, i.e. air, and the liquid electrolyte are present simultaneously. A description of the different types of air electrodes for zinc-air batteries is presented for example in the bibliographic article by V. Neburchilov et al., with the title “A review on air cathodes for zinc-air fuel cells”, Journal of Power Sources 195 (2010) pp. 1271-1291.
Batteries of the metal-air type function very well in discharge, but several problems during recharging have yet to be solved.
On the one hand, the air electrode is not designed to be used in the recharging direction. The air electrode has a porous structure and functions in the form of a volumetric electrode in which the electrochemical reaction takes place in the volume of the electrode, at the interface between a gas (the oxygen of the air), a liquid (the electrolyte) and a solid (the electrode and the catalyst). Thus, the interface between the air electrode and the liquid electrolyte is a so-called “triple-contact” interface where the active solid material of the electrode, the gaseous oxidant, i.e. air, and the liquid electrolyte are present simultaneously. The air electrode is usually composed of carbon particles with a large surface area such as Vulcan® XC72 marketed by Cabot. The surface area of the carbon can be increased by reaction with a gas, such as CO, prior to its integration in the air electrode. A porous electrode is then produced by agglomeration of the carbon particles using a fluorinated hydrophobic polymer such as FEP (fluorinated ethylene propylene) marketed by the company DuPont. Patent WO 2000/036677 describes such an electrode for a metal-air battery.
It is preferable to have a reaction surface area on the air electrode that is as large as possible, in order to have a current density relative to the geometric surface area of the electrode that is as high as possible. A large reaction surface area is also useful because the density of gaseous oxygen is low compared with a liquid. The large surface area of the electrode allows the reaction sites to be multiplied. Conversely, this large reaction surface area is no longer necessary for the reverse reaction of oxidation during recharging since the concentration of active material is much higher.
The use of an air electrode in the recharging direction to bring about an oxidation reaction and evolution of oxygen presents many drawbacks. The porous structure of the air electrode is fragile. It was observed by the inventors that this structure was destroyed mechanically by the evolution of gas when it was used to produce oxygen by oxidation of a liquid electrolyte. The hydraulic pressure generated within the electrode by the production of gas is sufficient to cause the bonds between the carbon particles constituting the air electrode to rupture.
It was also observed by the inventors that the catalyst added to the air electrode to improve the energy yield of the reaction of reduction of oxygen, such as manganese oxide or cobalt oxide, is not stable at the potential required for the reverse oxidation reaction. The corrosion of carbon in the presence of oxygen by oxidation of carbon is also accelerated at higher potentials.
Some inventors propose using a more resistant oxygen reduction catalyst coupled to an oxygen evolution catalyst in a bifunctional electrode composed of two electrically coupled layers, as described in patent U.S. Pat. No. 5,306,579. However, this configuration produces electrodes that nevertheless have a short service life and a limited number of cycles.
The degradation of the air electrode, when it is used to recharge the metal-air battery, greatly reduces the battery's service life. This is one of the main reasons for the low level of commercial development of electrically rechargeable metal-air accumulators.
A means for protecting the air electrode against degradation consists of using a second positive electrode, which is used for the oxygen evolution reaction. The air electrode is then decoupled from the oxygen evolution electrode and only the latter is used during the charging phase. For example, patent U.S. Pat. No. 3,532,548 of Z. Starchurski describes a zinc-air battery with a second auxiliary electrode used for the charging phase.
On the other hand, certain problems can also arise on the negative electrode side during electrical recharging of a metal-air battery, and quite particularly a zinc-air battery.
During recharging, the Zn2+ metal ions are reduced at the negative electrode and are deposited in their metallic form Zn once the potential at the level of this electrode is sufficiently negative. A uniform and homogeneous deposit of metal on the electrode is desired for ensuring good durability during the cycles of charging and discharging of this electrode.
It was found that, under certain conditions, the metal was deposited in the form of foam with little adherence to the surface of the electrode, and this foam could then become detached from the electrode, causing a loss of active material and consequently a loss of specific capacity of the battery. In other cases, it was found that the metal could also be deposited in dendritic form. These dendrites can grow until they reach the positive electrode during charging, causing an internal short-circuit, preventing recharging.
In an endeavour to solve these problems, and produce a homogeneous zinc deposit during recharging, certain solutions have already been proposed:                adding additives into the electrolyte (see for example C. W. Lee et al., “Effect of additives on the electrochemical behaviour of zinc anodes for zinc/air fuel cells”, Journal of Power Sources 160 (2006) 161-164, and C. W. Lee et al., “Novel electrochemical behavior of zinc anodes in zinc/air batteries in the presence of additives”, Journal of Power Sources 159 (2006) 1474-1477),        fitting a separator on the electrode (see for example H. L. Lewis et al., “Alternative separation evaluations in model rechargeable silver-zinc cells”, Journal of Power Sources 80 (1999) 61-65, and E. L. Dewi et al., “Cationic polysulfonium membrane as separator in zinc-air cell”, Journal of Power Sources 115 (2003) 149-152),        using a polymer hydrogel electrolyte as solid electrolyte (see for example C. Iwakura et al., “Charge-discharge characteristics of nickel/zinc battery with polymer hydrogel electrolyte” Journal of Power Sources 152 (2005) 291-294, G. M. Wua et al., “Study of high-anionic conducting sulfonated microporous membranes for zinc-air electrochemical cells”, Materials Chemistry and Physics 112 (2008) 798-804, and H. Ye et al., “Zinc ion conducting polymer electrolytes based on oligomeric polyether/PVDF-HFP blends” Journal of Power Sources 165 (2007) 500-508).        
Moreover, the Lawrence Berkeley Laboratory (LBL) and MATSI Inc. have sought to increase the porosity in the electrode in order to decrease the surface current densities responsible for the formation of dendrites, when they are high.